The present invention generally relates to the field of communication systems and more particularly, to achieving spatial diversity in a communication link having at least two transmitting antennas and at least one receive antenna while mitigating Doppler induced channel variations and multipath interference.
In a wireless communication system, a major design challenge is to maximize system capacity and performance in the presence of interference, and a time-varying multipath channel. A multipath channel is caused by the transmitted signal reflecting off objects near the transmitter and receiver and arriving at the receiver over multiple paths. Interference in a communication system can come from a variety of sources depending on the particular system deployment. If the system is in motion, then Doppler induced channel variations becomes a problem. Interference and multipath are major factors that limit the achievable performance and capacity of a communication system because both effects interfere with the ability of a communication receiver to properly decode the transmitted data.
In a multipath propagation channel, the transmitted signal propagates to the receiver over a finite number Lpaths of propagation paths, where each path has an associated time delay and complex gain. In such a channel, the communication receiver receives the superposition of Lpaths delayed, attenuated, and phase-shifted copies of the transmitted signal. The number of paths Lpaths and their time delays and phase shifts depends on the physical location of the various scattering objects (such as buildings, automobiles, and trees) in the immediate vicinity of the transmitter and receiver. The complex attenuation (magnitude and phase) of each path depends on the length of each path, as well as the material composition of any scatterers or reflectors encountered along the path.
The presence of multipath can severely distort the received signal. In a multipath environment, the multiple copies of the transmitted signal can interfere constructively in some portions of the occupied bandwidth. In other portions of the occupied bandwidth, the multiple copies can interfere destructively at the receiver. This signal duplication causes unwanted variations in the received signal strength over the bandwidth occupied by the signal. Furthermore, if the difference in the path delays of the various propagation paths is significantly greater than the duration of a transmitted information symbol, then intersymbol interference is present at the receiver. When intersymbol interference is present, the received signal is corrupted by prior transmitted symbols propagating over paths having delays relative to the shortest path that are longer than the duration of an information symbol. The demodulation process (the process of determining which information symbol was transmitted) becomes difficult in the presence of intersymbol interference.
In mobile communication systems, the presence of multipath causes rapid variations, or fading of the received signal strength at a receive device. The fading of the received signal can cause the received signal level to drop significantly to levels sometimes reaching 20 dB below the average received signal level. The rate of variation in the transmitted signal is related to the relative velocity of the transmitter and receiver, and to the scattering objects in the environment. These rapid variations in the signal strength can severely interfere with the ability of the receiver to decode the transmitted information.
When more than one antenna is employed at a receiver, the presence of multipath that arrives at the receive array from multiple different directions can cause the signal variations, or fading, on each of the receive antennas to be uncorrelated. In other words, the presence of angular multipath scattering can cause the channel responses between a transmitter and each of the receive antennas to be independent of each other. The result is that the signal level on one receive antenna might be totally unrelated to the signal level on another receive antenna. This means that when one antenna is in a fade (i.e., the signal level has dropped to a very low level), the signal level on another antenna might not be in a fade. Not only does multipath cause the channel response to vary in time (due to motion) and frequency (due to intersymbol interference, as discussed earlier), but multipath can also cause the channel response to vary in space (across the elements of an antenna array).
A well-known practice for combating fading is to use a technique called xe2x80x9cdiversity.xe2x80x9d In a generic sense, diversity is the practice of transmitting and/or receiving multiple copies of the signal and combining these copies in some optimal fashion. There are several methods of implementing diversity. One of the more popular forms of diversity is xe2x80x9creceive spatial diversity,xe2x80x9d in which multiple copies of the transmitted signal are received over multiple receive antennas and the outputs of the multiple antennas are combined in some optimal fashion, such as max-ratio combining. (Max ratio combining is the technique where the signals on the receive array are combined to maximize the signal to noise ratio at the output of the receive combiner.) The reason why exploiting receive spatial diversity improves receiver performance can be understood from the following line of reasoning. In a fading environment in which the fading processes are uncorrelated on the receive antennas, it is highly unlikely that all antennas of the receive array will simultaneously experience a severe drop in signal level. As a result, it is highly likely that at least one antenna element of the receive array is not in a fade, which means that at least one receive antenna is receiving a high-powered copy of the transmitted signal. The receiver will have difficulty when all receive antennas simultaneously go into a deep fade because no receive antenna can provide a high-powered version of the transmitted signal, and decoding errors will be highly likely. If the fading processes are all highly correlated, then all receive antennas will tend to go in and out of fades at the same time. When fades do occur in this case; the receiver will have difficulty decoding the signal because all antennas have faded simultaneously.
Another form of diversity similar to receive spatial diversity is xe2x80x9ctransmit spatial diversityxe2x80x9d also called xe2x80x9ctransmit diversity.xe2x80x9d Transmit spatial diversity, like receive spatial diversity, can be implemented in many ways. A primitive transmit diversity technique is to use multiple transmit antennas, where each transmit antenna simultaneously transmits the same signal. The receiver (either having a single receive antenna, or even multiple receive antennas) automatically receives the superposition of the two transmitted signals. When the channel responses between each of the transmit antennas and the receive antenna are uncorrelated, then a level of protection against fading can be achieved (for similar reasons as with receive diversity, as described above).
A properly designed transmit diversity scheme can be particularly valuable in a cellular system, especially when the subscriber device has only one receive antenna. In such a case, receive diversity is not an option on the downlink (base transmitting to the subscriber device) due to the single antenna design constraint that is often imposed by cost, size, and marketing requirements. Furthermore, many existing transmit diversity schemes suffer a performance loss when the propagation channel is rapidly varying. A transmit diversity scheme whose performance is not seriously degraded in a rapidly varying channel can also be particularly valuable in a mobile cellular system.
Thus, there is a significant need for a method and device for improved transmit diversity schemes so that the potential benefits of spatial diversity can be realized even when the receive device only has one receive antenna and even when the channel is rapidly varying.